I can't blame people for not buying them though, what's the point of a comic you can't even read? Even for collection purposes.
I have all but four of them, even though I can't read 'em. And all of the Viz as well. The reason why I collect the tankos is because the art is so much better in them than in scans, or in Viz. In the tankos, you see the art as Oda intended it to be seen – none of the little details (the things I like best about the story) are lost. Also, the lines are clear and sharp. And, as Alina said, there's extras like SBS and inside cover art, and of course the occasional hilarious pandaman cameo on the actual cover of the book, hidden beneath the dust jacket!
I collect the Viz for easy off-line reference and to show my support in an official, tangible way, for the attempt to bring, and keep, OP stateside. I figure that if they don't get enough support, they might drop the series, and I don't want to see that happen.
I'm lucky though -- I've got Kino bookstore within a day's drive, so I can get my OP tanko fix for $5 a piece and no additional shipping fees.